Graphene-nonlinear optical properties and modelocking capabilities

Abstract

In this thesis, graphene saturable absorber mirror's (SAM) potential for mode-locking ultrafast lasers has been demonstrated, in particular at 2 μm. The saturable absorption of graphene thin films (prepared via drop-casting of liquid-phase exfoliation graphene) was demonstrated at 1 μm, 1.2 μm and 2 μm, utilising the Z-scan technique. The graphene also reacted favourably with the different laser systems used, and in particular the different repetition rates (25 kHz, 100 kHz and 80 MHz). The nonlinear optical properties of graphene were determined to be ~-3,000 cm/GW and -0.15 cm2/GW for μ; and n2, respectively. These thin films were compared to chemical vapour deposition (CVD) graphene samples. The successful demonstration of saturable absorption in graphene at 2 μm allowed the group to move on to the development of mode-locking devices for ultrafast pulsed lasers operating at the same wavelength. Graphene SAMs were prepared via drop-casting and vacuum filtration of liquid-phase exfoliated (LPE) graphene, and by transfer of CVD prepared graphene, on silver mirrors. The nonlinear optical properties were examined with the I-scan technique at 2 μm. These were compared with a commercial semiconductor saturable absorber mirror (SESAM) from BATOP, and the graphene SAMs were proven to have similar characteristics. The vacuum-filtrated graphene SAM and the SESAM had very similar values for linear reflectance (63.6% to 64.6%), non-saturable losses (25% each) and modulation depth (~11% each). However, the graphene SAM needed a higher laser intensity to be saturated than the SESAM, according to the larger saturated intensity (Is). Considering that one advantage of ultrafast lasers are their high intensity, the larger Is is possibly acceptable in most practical applications. Stable mode-locking was achieved utilising the Graphene SAMs at 1.5 μm around 17 MHz.